The present invention relates generally to a microwave oven door sealing device. More particularly, the invention relates to such a sealing device which is relatively compact and which may either stand alone or be adapted to retrofit to a microwave oven already having a metal mesh gasket as a primary door sealing means. In the latter case, the present device serves both as a backup sealing means and to reduce electrical stress on the primary metal mesh gasket.
Microwave ovens for domestic use require some means for preventing leakage of the microwave energy from around the edges of the door when the door is closed. Various approaches to this energy sealing have been employed. Three in particular have been commercially employed. The first of these is direct electrical contact between the door and the oven liner. This is commonly known as a metal-to-metal seal, and may be effected for example by means of a metal mesh gasket such as is disclosed in U.S. Pat. No. 3,812,316 to Milburn. The second is a capacitive-type door seal wherein a flat surface portion around the liner and a surface portion of the door form the two plates of a capacitor. The capacitor presents nearly a short circuit to the microwave energy attempting to escape. Thirdly, various choke joint structures having cavities with an effective electrical length of either one-quarter or one-half wavelength are frequently used. Such chokes are based on a quarter wavelength transmission line impedance transformer principle, and function to present either a high impedance to block the passage of microwave energy or a low impedance to shunt the microwave energy, depending upon the particular application.
Microwave energy sealing devices such as these mentioned above have also been employed in various combination, one seal serving as a backup for the other. For example, a choke may serve as a backup for a primary metal-to-metal type seal. As another example, lossy gasketing materials such as conductive rubber or ferrite loaded rubber are commonly employed in combination with choke-type door seals. Such lossy materials absorb microwave energy, converting it to heat.
Particularly rigorous sealing requirements are found in combination ovens which are capable of both conventional and microwave cooking. Sealing problems are more difficult because the microwave sealing structure must be capable of withstanding the heat involved in the conventional cooking operation. This is particularly severe when microwave cooking is combined in a pyrolytic self-cleaning oven, because temperatures as high as 900.degree. F. may be reached during the self-cleaning process. One prior art example of a seal adapted for this type of oven is that disclosed in the above-mentioned U.S. Pat. No. 3,812,316 to Milburn. The Milburn gasket functions both to assure electrical contact between the oven door and the oven liner lip, and to seal the oven against the escape of heat, smoke and gases.
With respect to the microwave energy sealing function effected by the electrical contact, during microwave cooking substantial currents flow in the metal cooking cavity walls. These currents are a part of electromagnetic standing waves, known as "modes," supported within the cavity. A number of different modes are possible. Each of the modes has associated with it a particular pattern of current distribution in the cavity walls. For typical TE cooking mode, current maxima occur at a number of points along the interfaces between the wall edges. This includes the interfaces between the front edges of the cooking cavity walls and the door. As a result, particular electrical stresses are placed on the metal mesh gasket where it contacts the oven liner lip.
While such a metal mesh gasket normally provides a highly effective microwave energy seal, after a period of use a very small percentage of ovens employing such a gasket may exceed the very rigid standards governing permissible levels of microwave energy leakage from ovens. Such a failure might occur for example where a food spill or the like partially covers either the oven liner lip or the gasket, thereby preventing good electrical contact. This may be aggravated due to the relatively high current flowing through the gasket as a result of its location. An increase in resistance at this relatively high current location can result in heat-producing arcing which may permanently damage the gasket.
The present invention provides a compact and effective microwave energy sealing device which may either stand alone or be used in combination with another seal such as metal mesh gasket. When used in combination with a metal mesh gasket, the present invention not only serves as a backup should the metal mesh gasket completely fail, but additionally lessens the likelihood of a gasket failure by reducing the current flow through the gasket, and resultant electrical stress. Further, the construction of the device is such that it may be retrofitted to ovens already in use with minimum inconvenience and structural change to the oven. The device may be constructed of materials which are compatible with the high temperatures associated with pyrolytic self cleaning of an oven.